To obtain a device having a mechanism which adapts or optimizes itself according to the electric conductivity of water forming, specially, a water space to be observed as an echo imaging device which uses an electromagnetic wave propagated in plain water.

An antenna (e.g. dipole antenna) 1 placed in water is coupled with a transmitter receiver 2 in high frequency and with an electric conductivity measuring circuit 3 in direct current. A controller 5 obtains the observation result of the measuring circuit 3 and controls and modifies the operation elements of the transmitter receiver 2 to increase the observation frequency, increases the transmitted output, increase the gain of the receiver, or narrow the observation band width as the water has higher electric conductivity, i.e., has poorer quality and contains more impurities.